Our body accumulates the excess of nutrients we ingest in the form of fat. One of the reasons for this is that fat provides more than twice the energy per gram than sugar or proteins. Another reason is that fats are hydrophobic, meaning that they don’t mix with water. (Yes, that’s why oil stains are so hard to remove!). Since fats displace water, it’s easier to pack them and they weight less because they don’t have water molecules attached.
But though fat is the main nutrient reserve in our bodies, it’s not the only one. A small proportion of sugars are accumulated in the form of glycogen in your liver and muscles, acting as a short-term nutrient source. During fasting, your pancreas produces a hormone called glucagon that, among other things, promotes the breakdown of liver glycogen into small sugar molecules that are released into the bloodstream to keep sugar levels in the safe range. Similarly, when you engage in a rapid physical activity, such as sprinting, your muscles use their glycogen to feed the hungry muscular fibers. But glycogen stores are depleted very rapidly and only work for short-term needs. In fact, that’s glycogen depletion is the reason why you can’t sprint for a long time or too many times in a row.
All carbs are made of sugar, but not all carbs are equal (take a look at ‘Simple vs Complex Carbs‘). Fiber, for example, is a complex carb found in grains, fruits, vegetables, legumes and nuts, that our bodies can’t digest. This means that when you eat fiber, your sugar blood levels don’t increase (for more on fiber, check out ‘The importance of fiber‘). On the other hand, a processed food has plenty of added sugars that do increase your sugar blood levels. These are the ones you should target if you don’t want them to become extra pounds!
What happens when your blood has too much sugar is that some of it is used right away for energy, some is stored in the liver and muscles in the form of glycogen, and the rest is accumulated as fat in our bellies, hips and thighs.
Sugars and fats are made of carbon, hydrogen and oxygen. Proteins have the same elements plus nitrogen. The difference between these three types of molecules is how those atoms are organized in space, how they bind each other. Some of our cells have ‘tools’ to re-organize these molecules, and thus are able to transform sugar or proteins into fats. However, the opposite, transforming fats into sugars or proteins, is not possible – our cells lack the tools to do it!
The process by which the excess of sugar is transformed into fat is called ‘lipogenesis’. It takes place in the white adipose tissue, which is a fancy name for the places where you keep your extra pounds. The excess of sugars in your bloodstream stimulates the release of insulin from the pancreas, which promotes lipogenesis. The cells from your white adipose tissue take up the sugar and transform them into a type of fat molecules called ‘triglycerides’, which are then accumulated. That’s why too much sugar can make you fat (check out ‘Too Much Sugar‘ and ‘The 20 No-Sugar Days Diet‘ for more information).
And what about the proteins? While proteins can also be transformed into fats, in the absence of sugar there is little insulin, and thus the process of lipogenesis takes place at a much lower pace. Nevertheless, it should be borne in mind that insulin is also required for other things, such as building muscle (see ‘The Power of Protein‘), and hence keeping some natural occurring sugar in the diet is recommended.